A driver of a vehicle gazes at the front through a windshield, and performs driving while visually checking instruments on an instrument panel. That is, a line of sight moves to the front and lower instruments. If a driver can see instruments while looking at the front, there is no movement of the line of sight, and improvement of drivability can be expected. From this finding, a head-up display device has been developed and put to practical use (see, for example, Patent Literature 1).
As shown in FIG. 1 of Patent Literature 1, (the numerals in parentheses indicate the reference number described in Patent Literature 1, the same applies hereinafter), display light (L) is irradiated on the windshield (13) from the display device (12). The driver (14) seated in the driver's seat of the vehicle can visually recognize the display image (V) obtained by this irradiation through the windshield (13).
When the driver (14) replaces, the height of the eyes changes based on the physical difference (sitting height difference). A technique for changing the position of the display light (L) to cope with this change is disclosed in Patent Literature 1.
That is, the display device (12) shown in FIG. 2 of Patent Literature 1 includes a second reflector (40) capable of adjusting the emission angle of display light (L).
One form of the second reflector (40) is shown in FIG. 8 of Patent Literature 1. It comprises a concave mirror (41) for reflecting the display light and a resin mirror holder (60) for holding the concave mirror (41). In FIG. 8, on the side portion (61) of the mirror holder (60) positioned on the left side, there is provided a first shaft portion (62) for attaching the mirror holder (60) to the attached portion (54 a) of the boss portion (54) provided in the housing (50). The first shaft portion (62) is provided with a stress absorbing portion for absorbing a stress acting on the first shaft portion (62) when the first shaft portion (62) is attached to the attached portion (54 a).
Furthermore, an output gear (42 c) is fixed to the tip of the drive shaft (42 b) of the stepping motor (42 a) as an actuator, and a gear portion (63 a) is integrally formed on the distal end side of the second shaft portion (63) to mesh with the output gear (42 c). Here, the second shaft portion (63) is provided on the side portion (61) of the mirror holder (60) positioned on the right side in FIG. 8.
Then, when the stepping motor (42 a) is rotated in a state, in which the concave mirror (41) is attached to the housing (50) (that is, the first shaft portion (62) is bearing-held by the concave attached portion (54 a), the driving force of the stepping motor (42 a) is transmitted to the gear part (reference 48 a in FIG. 5 of Patent Literature 1) via the output gear (42 c). As the power (driving force) is transmitted to the gear portion (48 a), the gear portion (48 a) rotates about the rotation axis (reference symbol RA in FIG. 5 of Patent Literature 1), and the mirror holder (60) and the concave mirror (41) held by the holder (60) rotates (tilts) by a predetermined angle about the rotation axis (RA).
As described above, it is possible to change the position of the display light in accordance with the change in the height of the eyes of the driver.
However, when the mirror holder (60) and the concave mirror (41) rotate by receiving the driving force from the stepping motor (42 a), the first shaft portion (62) is bearing-held by the attached portion (54 a), and it is conceivable that stress concentrates on the mirror holder (60) located near the second shaft portion (63). This phenomenon can also occur even when the mirror holder (60) is made of a relatively hard resin. Further, this phenomenon can occur irrespective of the presence or absence of the stress absorbing portion (62 a).
As a result, twist (deformation) occurs in the mirror holder (60) located near the second shaft portion (63).
Also, from another point of view, a resistance force (resistance torque) due to friction is inevitably generated between the attached portion (54 a) and the first shaft portion (62). When the second shaft portion (63) located furthest from the attached part (54 a) is rotated by the stepping motor (42 a), the rotation angle of the second shaft portion (63) far from the attached portion (54 a) increases with respect to the first shaft portion (62) receiving the resistance. As a result, twist (deformation) occurs in the mirror holder (60) located near the second shaft portion (63).
With the occurrence of the twist (deformation), there is a problem that the display image (V) visually recognized by the driver (14) through the windshield (13) is distorted and the display quality of the display image (V) is impaired. There remains room for further improvement on this point.